Origin & Insertion

Most muscles have at least 1 Origin point and 1 Insertion point, which means a place on either end of the muscle which attaches it to bone. A well-known exception is your tongue, which is obviously not attached at one end! But how do you know which end is the Origin and which end is the Insertion?

 

Turns out it’s quite simple:

 Origin: The end attaching to the stable base which doesn’t move much or at all when the muscle is contracted – usually more Proximal (remember this from this post?)

Insertion: The end attaching to the ‘moving bone’ when the muscle is contracted – usually more Distal (Distal… Distant… remember?!)

 

Some muscles have multiple origin points, such as the Serratus Anterior on your superior 8-9 ribs, and others have 1 origin and 1 insertion such as the Semitendinosus (one of your hamstring muscles).

An easy example to look at is the Biceps: it has 2 origin points leading to 2 “heads” (where the name Biceps comes from) which merge and finish with 1 insertion.

 

You can see that the two tendons at the top which attach to parts of the Scapula are called the Origin because when you contract your bicep by bending your arm, your shoulder stays still – this is the proximal stable base. It doesn’t matter that your Scapula is able to move, it only matters that specifically when your bicep contracts, it stays stable.

 

In contrast, your Radius (your forearm bone) moves a lot! When you contract your bicep, your elbow bends and your forearm moves closer to your upper arm. So, the Biceps Brachii’s Insertion point is found on the distal bone, the Radius, which is affected the most by the contraction.
 

To make sure we’ve really got the hang of it, let’s look at a slightly less obvious example: Trapezius (Traps).

The Traps are a massive triangular pair of muscles that go much further down your back than the two bulging peaks across your shoulders. Each trap muscle is large enough to be divided into three sections: The Superior (upper) fibers, Middle fibers & Inferior (lower) fibers. Each of these three sections has multiple origin points and multiple insertion points. Without getting too deep into each point, we just want to determine how we know which is which – after all, the Traps move both the shoulder and the skull & neck!

 
But overall, the main role of the Traps is to move the shoulder. Out of the three sections, 2 are dedicated to the shoulder, and movement of the head and neck is only one part of the superior fiber’s job. So, it’s a fairly easy decision to call the medial (inside) part of the Traps the Origin because the stable base is the spine, and the moving part is the shoulder:

In this example, we’ve simplified things by saying that the Origin points track vertically from the base of the skull down the spine, the stable base, and the Insertion points all converge around the Scapula & Clavicle, the moving bones, but there are actually many different origin & insertion points which allow the traps to perform all it’s different roles. If you want to see them listed and in action, check out this little animation.

 
So, the next time you’re wondering what the Origin or Insertion of a muscle is, just think: which end stays still and which end moves?